The Cascade Range is part of a vast mountain chain that spans for over 500 miles, from Mount Shasta, in northern California to British Columbia in the north.

The beautiful North Cascade Range, located in northwestern Washington State, has some of the most scenic, and geologically complex mountains in the United States. The sharp peaks, and rugged terrain of the northern portion of the range have an average elevation of 7,000 feet and are home to numerous alpine glaciers. The Range receives an average of 80 inches of rain annually, and as a result, it is densely vegetated.

The geologic history of the North Cascade Range is a complicated puzzle that records over 400 million years of various rocks and terranes that have been scraped off and smashed together, folded and buried, faulted and moved, finally making their way to their present day position in western Washington. After the chaotic assembly of the various terranes, a chain of volcanoes grew and erupted, covering the already complex geology with lava and ash. Volcanism continues to this day. Mount Baker and Glacier Peak are the two youngest volcanoes in the Range—they stand out above the rest, reaching to over 10,000 feet.

Terrane Accretion

The rocks we see today in the North Cascades are the result of a complicated geologic history. Prolonged subduction along the western margin of North America has resulted in the docking (accretion) of many exotic blocks of land (terranes). The older bedrock of the North Cascades can be generally divided into three main parts, or “domains”. From west to east these are: the Northwest Cascades Thrust System, Cascade Crystalline Core, and Methow Basin domains. Each domain in turn contains numerous accreted terranes. Contacts between the different terranes are usually large-scale faults and are frequently zones where igneous plutons intruded during accretion.

Domains, structural blocks, and major fault zones within the North Cascade Range. Modified from Haugerud and Tabor (2009)

Northwest Cascade Thrust System

Cascades Crystalline Core

Methow Basin

The Northwest Cascades Thrust System (NWCS) domain consists of a folded stack of terranes: Nooksack, Chilliwack River, Bell Pass Mélange, Easton, and the Yellow Aster Complex. The oldest rocks are found on top (the opposite of what you’d expect in typical sedimentary layering) because the terranes of the NWCS Domain have been thrust over each other and then folded into a large arch. Much of the NWCS domain was eventually eroded away, exposing the oldest rocks at the bottom of the thrust package. The Chuckanut Formation was deposited on top of this thrust package, and the entire sequence of rocks was later intruded by volcanoes and plutons.

The Cascade Crystalline Core Domain rocks are made up of the accreted terranes of the Insular and Intermontane belts as well as oceanic rocks, continent-derived rocks, and rocks that formed in the ancient volcanic arc. Beginning about 90 million years ago, all of the rocks in the Cascade Crystalline Core Domain were metamorphosed due to deep and prolonged burial. These rocks make up some of the most beautiful alpine scenery in all of Washington.

The Cascade Crystalline Core Domain is made of the following terranes: Napeequa Schist, Chiwaukum Schist, Cascade River Complex, Swakane biotite gneiss, and assorted plutons. These terrains vary in rock type (metamorphosed oceanic rocks, arc volcanics, sediments, pieces of the mantle, and old continents), and each original rock type has a different age (ranging between 200 and 570 million years old!) and geologic history of its own. Many plutons were intruded into the buried and accreted terranes during metamorphism, such as the Mount Stuart Batholith. The intruding magma supplied some of the heat that was needed for recrystallization and metamorphism.

The Methow Basin Domain is located on the east side of the North Cascade Range. This domain is composed of the Methow stratigraphic sequence and the Hozomeen Group. The Methow Basin Domain originated on the ocean floor between 220 and 350 million years ago. The ocean floor was made of layers of basalt, sandstone, mud, chert, and other sediments that were eroded off the continent into the ocean. The sediments and basalt were subsequently uplifted, and faulted, and thrust eastward in a series of fault-bend folds.

Cascade Volcanism

The Cascade Arc has been erupting a chain of volcanoes over the last 40 million years along the crest of the modern day Cascade Range. During this time the Cascade Arc erupted to cover the older metamorphic, mélange, and sedimentary rocks with lava and ash. The South Cascades record this older period of volcanic eruption, as most of the rocks exposed there are from this time; the extensive Ohanapecosh Formation is a good example of this. The volcanoes that erupted in the North Cascades during this time period have been eroded away because the North Cascades are being uplifted at a greater rate than the south Cascades. However, the plutonic roots of these ancient volcanoes remain, and they outcrop as numerous large batholiths such as the Chilliwack batholith, the Snoqualmie batholith, and the Cloudy Pass pluton.

Cascade volcanism began within the last 5 to 7 million years, and the modern-day volcanic arc is still active, extending from Mount Garibaldi in British Columbia in the north to as far south as Mount Lassen in California. Two volcanoes lie within the North Cascades: Mount Baker and Glacier Peak.

Mount Baker is a stunning, glacially covered, 10,781 feet tall, very young (geologically speaking) volcano. It is only ~50 to 70,000 years old, and is the latest installment of the Chilliwack Batholith. The most recent eruption of Mount Baker was in 1880, although there were signs of unrest and fumarole activity in 1975. Steam and gas still issue from both Sherman Crater and the Dorr fumarole field on the northeast flank of the volcano today. Mount Baker holds the world record for most snowfall in a single season—95 feet in 1999! For more information about this volcano, visit our Volcanoes and Lahars page or the Mount Baker Volcano Research Center.

Glacier Peak is the most remote of the Cascade volcanoes in Washington State, and is the fourth tallest of all peaks in state at 10,525 feet. As the name would imply, Glacier Peak is covered by more than a dozen glaciers. Glacier Peak has produced some of the most explosive eruptions in the U.S. and has erupted multiple times in the last 15,000 years. The most recent eruption was only ~300 years ago. Although Glacier Peak is tucked away, the explosive eruptions and the resulting lahars (debris mudflows that originate on the slope of a volcano) generated from Glacier Peak are a hazard along the river valleys draining its flank.

Geologic mapping has documented the extent of previous lahar runout in the Skagit and Stillaguamish River valleys. While Glacier Peak has shown no sign of eruption in the last few decades, the lahars deposits in the river valleys from past eruptions are a reminder of the hazard Glacier Peak poses to the communities living in the valleys adjacent to the volcano.

We know from geologic mapping that since the last ice age, several eruptive episodes resulted in catastrophic deposition along channels draining from Glacier Peak.

Major Faults

Domains, structural blocks, and major fault zones within the North Cascade Range. Modified from Haugerud and Tabor (2009)

Three main faults divide the domains, as well as a number of smaller faults that locally offset the different terranes: the Straight Creek fault, the Ross Lake fault zone, and the Pasayten fault. These faults have been slicing and shifting the various terranes northward over the past 100 million years.

The Straight Creek fault is a right lateral strike slip fault; it has offset rocks as much as 63 miles northward on the west side of the fault. The Darrington-Devils Mountain fault zone splays off the Straight Creek fault to the west and separates the North Cascades Domains to the north from the mélange belt to the south.

Terrane accretion was followed by an episode of extension and massive erosion and deposition (~45 to 50 million years ago). Rivers and streams washed over the older metamorphosed rocks and plutons, and then deposited sediment in extensional basins formed from the stretching and pulling of the crust. The extension made room for granitic magma to intrude along faults.

Glaciation and Erosion

About 25,000 to 13,000 years ago, during the last ice age, much of the North Cascade Range was covered by glaciers. Some alpine glaciers originated on the peaks of the tallest mountains and remain there today. During this time, the Cordilleran ice sheet was advancing south from British Columbia and most of the range was covered by a thick sheet of ice—only the tallest peaks rose above the ice. As the ice sheet retreated, massive amounts of water ran off the glacier, eroding the mountains, carving U-shaped valleys, creating lakes, and depositing massive amounts of glacial sediment.

Thickness and extent of glacial ice in the North Cascade Range during the last ice age. Modified from Haugerud and Tabor (2009)

Mineral Deposits

The northeastern Cascade Range is known for mineral deposits of various geologic ages. Former mining camps and prospects are scattered throughout the northwestern Cascades, and primarily recovered copper, silver, and gold.

Faults associated with shallow intrusions contain copper, molybdenum, and tungsten, and are great places to find quartz and pyrite crystals. The Spruce Ridge Claim in the Snoqualmie batholith in King County is a world-class mineral locality. This batholith also contains a large porphyry-copper deposit. The state's largest copper producer, the Holden Mine in Chelan County, produced 207 million pounds of copper along with zinc, silver, and gold from 1938 to 1957. The largest copper deposit in the state is at Miners Ridge in Snohomish County. There, drilling in the Cloudy Pass pluton has outlined a porphyry copper resource of 1.9 billion tons. However, this deposit will not be developed because it is within the protected Glacier Peak Wilderness Area.

The Cannon Mine in Wenatchee closed in 1994, but at one point was the second largest underground gold mine in the country, producing over 1.4 million ounces of gold and 2 million ounces of silver. The mine was developed in an epithermal deposit, which formed in the Earth's crust as volcanic rocks were intruded.

The spectacular Twin Sisters are made up of dunite, an ultramafic rock that contains the largest olivine reserves in the United States.